1. Field of the Invention
The present invention relates to a cordierite honeycomb structural body used as a catalyst carrier for an exhaust gas purification catalyst in an internal combustion engine such as an automobile engine, and to a process for producing that cordierite honeycomb structural body.
2. Description of the Related Art
Accompanying the increasing severity of automobile emissions controls in recent years, there has been a demand for reducing hydrocarbon emissions immediately after engines are started through more rapid activation of exhaust gas purification catalysts. Since reducing the heat capacity of the honeycomb structural body serving as the catalyst carrier is effective for more rapid activation of the catalyst, it is therefore necessary to reduce the thickness of cell walls to achieve this.
In general, a honeycomb structural body made of cordierite is suitably used for catalyst carriers. For example, as described in Japanese Unexamined Patent Publication (Kokai) No. 7-138077, a cordierite honeycomb structural body is produced by adding molding aids such as binder, lubricant and humectant, as necessary, to a cordierite-converted starting material, followed by kneading with a liquid medium and extrusion molding the resulting molding material. A mold for extrusion molding having honeycomb-shaped extrusion channels and a feed path through which molding material is supplied to the extrusion channels is normally used for extrusion molding of the honeycomb structural body. Molding material supplied to the intersections of the extrusion channels from the feed path is pressed together in the extrusion channels to obtain a honeycomb-shaped molded body.
However, when molding is attempted by reducing the thickness of the cell walls of the cordierite honeycomb structural body to less than that of the prior art to, for example, 100 xcexcm or less, the width of the extrusion channels becomes less and the resistance present when the molding material passes through the extrusion channels increases significantly. Consequently, the surfaces of the outer wall or cell walls of the formed honeycomb structural body are finely split, or molding material is unable to adequately fill the insides of the extrusion channels due to frictional resistance between the mold walls and molding material, whereby the molding material is extruded without being pressed together, thereby leading to the problem of preventing the formation of a honeycomb shape.
On the other hand, it is also possible to soften the molding material to reduce the frictional resistance. Although fluidity is improved by softening the molding material, the shape retention of the extrusion molded honeycomb structural body decreases, resulting in the problem of it being easily deformed by its own weight or external force.
Therefore, the object of the present invention is to obtain a cordierite honeycomb structural body that makes it possible to reduce the thickness of the cell walls and reduce heat capacity by improving the fluidity of the molding material during passage through the extrusion mold while maintaining the shape retention of the molding material.
The first aspect of the present invention comprises a cordierite honeycomb structural body comprising addition of a binder to a cordierite-converted starting material, kneading and extrusion molding of the resulting molding material. The above-mentioned binder is a water-soluble cellulose ether that contains methoxyl groups substituted for the cellulose hydroxyl groups within the range of a substitution rate of 27.5 to 31.5%, does not contain hydroxypropoxyl groups and hydroxyethoxyl groups in an amount greater than the amount of unavoidable impurities (0.1%), and has a viscosity in a 2 wt % aqueous solution at 20xc2x0 C. of less than 8000 centipoise. The amount of this water-soluble cellulose ether added is within the range of more than 3 wt % and not more than 10 wt % relative to the amount of cordierite-converted starting material.
Use of the prescribed amount of the above-mentioned specific water-soluble cellulose ether as binder makes it possible to improve fluidity during passage through the mold for extrusion molding while maintaining the shape retention of the molding material. This is the result of improving fluidity by decreasing frictional resistance, since the distance between starting material particles increases as a result of a substance like the above-mentioned specific water-soluble cellulose ether having low frictional resistance being inserted between the starting material particles. In addition, since the above-mentioned specific water-soluble cellulose ether is highly hydrophilic and can convert the starting material to a plastic substance even with a small amount of water, it thus becomes a hard molding material and enhances shape retention. Accordingly, a high-quality cordierite honeycomb structural body having low heat capacity can be realized which allows reduction of the thickness of cell walls without lowering moldability.
Another aspect of the present invention is characterized in that, in the production of a cordierite honeycomb structural body by adding a binder to a cordierite-converted starting material, kneading and extrusion molding the resulting molding material, a water-soluble cellulose ether is used as the above-mentioned binder, the water-soluble cellulose ether containing methoxyl groups substituted for the cellulose hydroxyl groups within the range of a substitution rate of 27.5 to 31.5%, not containing hydroxypropoxyl groups and hydroxyethoxyl groups in an amount greater than the amount of unavoidable impurities (0.1%), and having viscosity in a 2 wt % aqueous solution at 20xc2x0 C. of less than 8000 centipoise, and this water-soluble cellulose ether is added within the range of more than 3 wt % and not more than 10 wt % relative to the amount of cordierite-converted starting material.
According to the above-mentioned process, a high-quality cordierite honeycomb structural body with low heat capacity can be obtained which allows reduction of the thickness of cell walls while realizing both shape retention and fluidity of the molding material for the same reason as in the above-mentioned first aspect.
The above-mentioned water-soluble cellulose ether in either the above-mentioned first or second aspect of the present invention preferably does not contain substitution groups in an amount greater than the amount of unavoidable impurities (0.1 wt %) other than methoxyl groups. Preferably, water-soluble cellulose ether should be used which contains as functional groups only methoxyl groups substituted for the hydroxyl groups of cellulose, since the effect of enhanced shape retention and fluidity is correspondingly higher.
The thickness of the cell walls of the above-mentioned cordierite honeycomb structural body in either the above-mentioned first or second aspect of the present invention is preferably 100 xcexcm or less. If the cell wall thickness is 100 xcexcm or less, the above-mentioned effects of the present invention become prominent, resulting in a high level of usage value.